1. Field of the Invention
The present invention relates to a focusing method. More particularly, the present invention relates to a focusing method suitable for an image capturing apparatus using in an environment of low brightness.
2. Description of Related Art
FIG. 1 shows a basic architecture in a digital camera. Referring to FIG. 1, the digital camera 100 at least includes a lens 104, an image sensing unit 106, an analog/digital converting unit 108, and a digital image processing apparatus 110. An image 102 of an object is converted into a series of voltage signals 107 through the lens 104 by the image sensing unit 106 in a manner of scanning, and then, the voltage signals are converted into digital image sensing data 109, i.e., generally called frame image data, by the analog/digital converting unit 108, such that the image is processed in the digital image processing apparatus 110, and finally, the image captured by the user is obtained. As for the focusing method of the digital camera 100, the obtained image data is used for performing calculations, so as to obtain a focus position.
However, under the environment of low brightness, the common digital camera is restricted by the sensitive capability of the image sensing unit 106, and thus, it usually cannot be focused effectively. In view of this disadvantage, the digital camera generally adopts a focus assist light or a flash light to increase the environmental brightness, so as to improve the focusing performance under the environment of low brightness. Even so, with the image processing method on the digital image processing apparatus 110 in the conventional art, the digital camera still cannot obtain the suitable focus under the environment of low brightness, and the reasons are described as follows.
Referring to FIG. 2, it mainly shows a basic architecture of a digital image processing apparatus 110. In FIG. 2, the digital image processing apparatus 110 includes a dynamic random access memory (DRAM) 202, an interface module 204, an image pipeline module 206, a statistics module for contrast value 208, an image scaling module 210, and a display module 212. The DRAM 202 can be used to register the data required to be processed by the interface module 204, the image pipeline module 206, the image scaling module 210, and the display module 212.
When the user previews the images on the digital camera, after the interface module 204 receives the digital image sensing data 109, the interface module 204 itself or the image pipeline module 206 generally performs a horizontal/vertical down sample process on the digital image sensing data 109, so as to reduce the data to a suitable display size. Then, the statistics module for contrast value 208 is used to count the contrast value in statistics, so as to perform the automatic focusing process. Next, the image pipeline module 206 further processes the image, for example, performs a color filter array interpolation, so as to separate the voltage signals into three primary color signals, and performs a color space transfer. Then, the image scaling module 210 is used to scale the image to a size required by a screen 214, and then output the image to the display module 212 that displays the image on the screen 214.
The so-called down sample process, taking the horizontal down sample process as an example, the data of 2048×300 pixels is reduced to 640×300 pixels, for example, as shown in FIG. 3, A size is reduced to B size. FIG. 3 is a schematic view of the horizontal down sample process. The data of 2048×300 pixels contains 300 batches of horizontal scanning line data for the horizontal direction, and contains 2048 batches of vertical scanning line data for the vertical direction. As for the vertical down sample process and the vertical and horizontal down sample process, the principles thereof are the same as that of the horizontal down sample process, so they are not described herein any more.
The digital image sensing data 109 is arranged and presented in a Bayer pattern put forward by Doctor Bayer, and as shown by Number 402 of FIG. 4, which is usually called raw data. Raw indicates an original record after the image sensing unit (as shown by 106 in FIG. 1) senses the light, and completely records the color level generated after the image passes through a Bayer-pattern filter. If the down sample process is performed on the digital image sensing data 109, for example, the horizontal down sample process is performed on the vertical scanning line data 404, 406, and 408 of FIG. 4, when a vertical scanning line data is formed, the resolution is reduced, and the original record is damaged.
Therefore, in the case that the environmental brightness is sufficient, the digital camera obtains the contrast value by using the data after the down sample process while changing the focus of the lens. At this time, a focus curve is obtained, and the focus curve is sufficient for the statistics module for contrast value (as shown by 208 of FIG. 2) to obtain a position where the contrast value is the highest in statistics, so as to obtain the optimal focus, as shown in FIG. 5. FIG. 5 shows a focus curve when the environmental brightness is sufficient. However, if the environmental brightness is not sufficient, and the curvature of the whole focus curve becomes indistinct due to a reduced signal to noise ratio (SNR), it is hard to determine the optimal focus point, as shown in FIG. 6. FIG. 6 shows a focus curve when the environmental brightness is insufficient. If the noise effect is further considered, the more actual focus curve is shown in FIG. 7. FIG. 7 shows a focus curve with noises.
In addition, in the conventional art, the exposure time of the same batch of digital image sensing data (i.e., the same frame) is prolonged, for example, the exposure time is prolonged from the original 1/30 seconds to 1/15 seconds, so as to increase the focusing capability of the digital camera in the environment of low brightness, and the effect is not desirable. Therefore, it is an urgent problem to be solved by designers and manufacturers of the digital camera about how to increase the focusing capability of the digital camera in the environment of low brightness, which not only involves the quality of the image captured by the digital camera, but also greatly affects the consumers' purchasing desire.